Lake Tahoe’s famed clarity has long been used to gauge the health and changing conditions of Big Blue.

Unlike in recent years when researchers were able to point to a dominant factor affecting lake clarity like drought or higher-than-average precipitation, 2019 saw a range of influences on Tahoe, including lake mixing for the first time in several years, sediment, algae, and climate warming. Those factors, according to the University of California, Davis Tahoe Environmental Research Center, combined to cause a roughly 8-foot decrease in average clarity from the previous year’s 10-foot improvement.

The research center on Wednesday released its 2019 Lake Tahoe clarity report, which stated the average annual value for clarity at the lake for the year dropped 62.7 feet. The lake’s average annual clarity in 2018 was 70.9 feet. The lowest recorded average value for Tahoe was recorded in 2017 when the lake’s average annual clarity was 60 feet.

“On its face, this data is startling, but not unexpected,” said League to Save Lake Tahoe CEO Darcie Goodman Collins in a statement. “After an encouraging 10-foot gain in clarity from 2017 to 2018, the losses from 2018 to 2019 reveal how strongly Tahoe’s blue is tied to the effects of the climate crisis and influenced by the Lake ‘turning over’ or mixing all the way to the bottom. It also spotlights the need for everyone of us — public agencies, environmental organizations, local businesses, visitors and residents — to do all we can to Keep Tahoe Blue.”

Clarity at Lake Tahoe is measured with a 10-inch white Secchi disk, which is lowered into the water until it can no longer be seen. In 2019, researchers conducted 28 individual readings at the long-term index station maintained by UC Davis.

Year-to-year fluctuations for Tahoe are common, according to the Tahoe Environmental Research Center, which said a truer picture of clarity is better indicated by a five-year running mean. Since the research center’s 2015 report, Lake Tahoe’s average clarity is 67.3 feet.

2019 decrease in clarity

Scientists at UC Davis determined there were a number of factors for the decrease in clarity seen in 2019.

As part of the drop in clarity, researchers identified higher-than-average precipitation in 2019; the lake mixing all the way to the bottom for the first time in 8 years, which brought accumulated nutrients to the surface, aiding algal growth last spring; higher than average loads of particles washing in from the surrounding watershed; the presence of natural tiny algal cells called Cyclotella, which resulted in 36 feet of clarity in May 2019; and warming of lake surface waters, which kept fine particles from the watershed afloat near the water surface in June and July, clouding transparency during summer months.

The lake’s mixing for the first time in 8 years resulted in the highest clarity value of the year at 112 feet on Feb. 19, 2019. When deep mixing occurs, according to scientists at UC Davis, it brings clear water to the surface, immediately improving clarity. The nutrients that are also moved up the water column, however, can result in algal blooms later in the year.

In winter of 2018-19 the average Secchi depth was 81 feet, based on eight readings between December and March. The long-term average Secchi depth for winter months is 84 feet.

During the summer months of 2019, the average Secchi depth was 53 feet, based on 10 readings from June to September. The 53-foot summer average is the fourth lowest on record. The lowest recorded average Secchi depth for summer months was 51 feet in 2008.

Invasive Mysis shrimp

Among the several factors that are attributed to the decrease in Tahoe’s clarity, invasive Mysis shrimp have recently taken center stage in research done by the Tahoe Environmental Research Center.

“With lake warming impacting the recovery of Lake Tahoe’s clarity, we’re looking for ways that can mitigate climate impacts. Approaches such as the removal of the invasive Mysis shrimp are showing great promise for clarity improvement in the next few years,” said Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center in a news release. “The environmental services that the natural Tahoe ecosystem provided are now being better recognized.”

Mysis shrimp were introduced to Tahoe from 1963-1965 by the California and Nevada Departments of Fish and Game due to the belief that their population would provide plentiful food supply for lake trout.

Once the shrimp became established, however, they did not successfully supplement the food supply for sport fishes, and instead caused unexpected negative impacts on the lake, damaging clarity levels, and nearly eradicating the native zooplankton, Daphnia and Bosmina, which act as the lake’s natural cleaners.

When shrimp aren’t present, the Daphnia and Bosmina are able improve the lake’s clarity by consuming fine particles and Cyclotella in the water column, turning them into fecal pellets, which then sink to the bottom of the lake.

The Tahoe Environmental Research Center is nearing the end of a two-year pilot project, trawling for the shrimp at night in order to find an effective means of removing enough Mysis to improve lake clarity.

Decades of work

UC Davis has been conducting continuous monitoring of Lake Tahoe since 1968. That research has been used to inform policymakers and stakeholders on strategies to protect the lake and stabilize its decline in clarity.

Since the late 1960’s through the turn of the century, scientists have observed a decline in Tahoe’s clarity. During the past decade, however, scientists have reported the rate of decline in clarity has eased, while also recording an improvement in average clarity during the winter months.

“Understanding why summer clarity continues to decline is our highest priority, and we continue to work closely with the science community to understand how to reverse that decline,” said Joanne Marchetta, executive director of the Tahoe Regional Planning Agency, which funds the clarity report, in Wednesday’s news release. “We remain committed to the lake’s restoration in the face of the urgent threats of climate change and invasive species.”

Justin Scacco is a reporter for the Sierra Sun. Contact him at jscacco@sierrasun.com or 530-550-2643.